The objective of this study is to investigate the permeability of the intestinal microvasculature to plasma proteins and small lipid insoluble substances and to determine the effect of various hormones on the intestinal absorption of Na, Cl and glucose. An isolated canine or feline intestinal loop will be autoperfused, the venous outflow cannulated to measure flow, the lymphatic outflow cannulated and the loop placed into a plethysmograph to monitor intestinal volume changes. For the permeability studies, two different approaches will be used. 1) A thermodynamic analysis will be conducted in the isolated intestinal preparation and the reflection coefficients of several small lipid insoluble substances will be measured. 2) The concentration of the various protein fractions in lymph (alpha1, alpha2, beta1 albumin and gamma globulins) will be measured and compared to the plasma concentration of each fraction. From this data, an effective (equivalent) pore model of the intestinal microvasculature can be evaluated. Various vaso-active drugs will be used to evaluate the differences between mucosal and submucosal exchange vessels (histamine, epinephrine, norepinephrine, angiotensin, etc.) by measuring lymph to plasma ratios of the various protein fractions and the reflection coefficients of several small lipid insoluble molecules for each vaso-active drug. The results will be compared with those obtained in the normal intestinal preparation and with those measured with elevated venous pressure. In addition, the various capillary and tissue forces (capillary pressure, plasma colloid osmotic pressure, lymph colloid osmotic pressure, and tissue pressure) and flows (lymph flow, protein flux) will be studied for each experimental condition. The transport studies will be conducted to determine how the various hormones affect intestinal absorption either by: 1) a direct effect that changes the active transport system or by 2) an indirect effect that changes tissue and capillary forces. These studies will allow us to build a more comprehensive model of the capillary-tissue-intestinal transport system because each force and flow will be measured in each preparation and related to changes in capillary forces or intestinal active transport systems.